Correct the Explanation of PATH Variable Lookup (#2975)

This PR corrects a small but important detail in the documentation regarding how the system searches for executable files. The original text incorrectly stated that the operating system performs this search. The corrected text clarifies that it is actually the shell that performs the search.

README.md

@@ -86,7 +86,7 @@ to the correct Python installation.
 
 ### Understanding PATH
 
-When you run a command like `python` or `pip`, your operating system
+When you run a command like `python` or `pip`, your shell (bash / zshrc / ...)
 searches through a list of directories to find an executable file with
 that name. This list of directories lives in an environment variable
 called `PATH`, with each directory in the list separated by a colon: