Remove comment suggesting the undefined sanitizer is a valid option

Also makes come structural improvements to how the local instructions
for running OSS-Fuzz are presented now that only the single `address`
sanitizer is a valid option.

The `undefined` sanitizer was removed from GitPython's `project.yaml`
OSS-Fuzz configuration file at the request of OSS-Fuzz project reviewers
in google/oss-fuzz#11803.

The `undefined` sanitizer is only useful in Python projects that use
native exstensions (such as C, C++, Rust, ect.), which GitPython does
not currently do. This commit updates the `fuzzing/README` reference to
that sanitizer accoirdingly.
See:
- google/oss-fuzz@b210fb2
- google/oss-fuzz#11803 (comment)

Author: DaveLak

fuzzing/README.md

@@ -129,26 +129,15 @@ This approach uses Docker images provided by OSS-Fuzz for building and running f
 comprehensive features but requires a local clone of the OSS-Fuzz repository and sufficient disk space for Docker
 containers.
 
-#### Preparation
-
-Set environment variables to simplify command usage:
-
-```shell
-# $SANITIZER can be either 'address' or 'undefined':
-export SANITIZER=address
-# specify the fuzz target without the .py extension:
-export FUZZ_TARGET=fuzz_config
-```
-
-#### Build and Run
+#### Build the Execution Environment
 
 Clone the OSS-Fuzz repository and prepare the Docker environment:
 
 ```shell
 git clone --depth 1 https://github.com/google/oss-fuzz.git oss-fuzz
 cd oss-fuzz
 python infra/helper.py build_image gitpython
-python infra/helper.py build_fuzzers --sanitizer $SANITIZER gitpython
+python infra/helper.py build_fuzzers --sanitizer address gitpython
 ```
 
 > [!TIP]
@@ -160,16 +149,25 @@ python infra/helper.py build_fuzzers --sanitizer $SANITIZER gitpython
 > Then running this command would build new or modified fuzz targets using the `~/code/GitPython/fuzzing/fuzz-targets`
 > directory:
 > ```shell
-> python infra/helper.py build_fuzzers --sanitizer $SANITIZER gitpython ~/code/GitPython
+> python infra/helper.py build_fuzzers --sanitizer address gitpython ~/code/GitPython
 > ```
 
-
 Verify the build of your fuzzers with the optional `check_build` command:
 
 ```shell
 python infra/helper.py check_build gitpython
 ```
 
+#### Run a Fuzz Target
+
+Setting an environment variable for the fuzz target argument of the execution command makes it easier to quickly select
+a different target between runs:
+
+```shell
+# specify the fuzz target without the .py extension:
+export FUZZ_TARGET=fuzz_config
+```
+
 Execute the desired fuzz target:
 
 ```shell