fix sphinx #12308: typo in extension in helper .htp -> .hhp

[AJPfleger]

Taken from the linked issue (from another sphinx-repo):

class HTMLHelpBuilder in init.py names an incorrect file extension in the string of the variable epilog for the correct generated HTMLHelp project file.

Incorrect is the part .htp because .hhp is meant.

closes:

• Extension sphinxcontrib-htmlhelp has typo sphinx#12308