WebView: fix broken references to #onShowFileChooser

WebChromeClient#onShowFileChooser was incorrectly referred to as
"showFileChooser" in two locations. Now these locations use the correct
name and the spot using "<code>" has been fixed to use "{@link}".

This also fixes a grammar error in the docs.

Bug: 65213653
Test: make docs (and manually verify the link works)
Change-Id: I17ba8ac3f76371a8b8a12b998ce4c956b6237119