doc: change pyobjc link in macos build instruction (electron#18471)

Author: xtx1130

docs/development/build-instructions-macos.md

@@ -29,7 +29,13 @@ $ brew install python@2 && brew link python@2 --force
 If you are using Python as provided by Homebrew, you also need to install
 the following Python modules:
 
-* [pyobjc](https://pythonhosted.org/pyobjc/install.html)
+* [pyobjc](https://pypi.org/project/pyobjc/#description)
+
+You can use `pip` to install it:
+
+```sh
+$ pip install pyobjc
+```
 
 ## macOS SDK