Change short help messages to imperative voice

Unify the output of `restic help`.

doc/man/restic-forget.1

@@ -5,7 +5,7 @@
 
 .SH NAME
 .PP
-restic\-forget \- forget removes snapshots from the repository
+restic\-forget \- remove snapshots from the repository
 
 
 .SH SYNOPSIS

doc/man/restic-tag.1

@@ -5,7 +5,7 @@
 
 .SH NAME
 .PP
-restic\-tag \- modifies tags on snapshots
+restic\-tag \- modify tags on snapshots
 
 
 .SH SYNOPSIS