DOC: Add conda uninstall pandas to contributing guide (pandas-dev#25490)

* fix pandas-dev#25487 add modify documentation

Author: yehia67

doc/source/development/contributing.rst

@@ -178,6 +178,7 @@ We'll now kick off a three-step process:
    # Create and activate the build environment
    conda env create -f environment.yml
    conda activate pandas-dev
+   conda uninstall --force pandas
 
    # or with older versions of Anaconda:
    source activate pandas-dev