DOC: fix the editable install command (pandas-dev#28445)

Author: jorisvandenbossche

doc/source/development/contributing.rst

@@ -208,7 +208,7 @@ We'll now kick off a three-step process:
 
    # Build and install pandas
    python setup.py build_ext --inplace -j 4
-   python -m pip install -e --no-build-isolation .
+   python -m pip install -e . --no-build-isolation
 
 At this point you should be able to import pandas from your locally built version::
 
@@ -252,7 +252,7 @@ You'll need to have at least python3.5 installed on your system.
 
    # Build and install pandas
    python setup.py build_ext --inplace -j 4
-   python -m pip install -e --no-build-isolation .
+   python -m pip install -e . --no-build-isolation
 
 Creating a branch
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