glossary: improve description of SHA-1 related topics

glossary: improve description of SHA-1 related topics

The name of the hash function is "SHA-1", not "SHA1".

Also to people who look up "object name" in the glossary,
the details of which hash function is applied on what to
compute "object name" is not important but the fact that the
name is meant to be an unique identifier for the contents
stored in the object is.

Signed-off-by: Thomas Ackermann <redacted>
Signed-off-by: Junio C Hamano <redacted>