Add goto-instrument's synthesize-loop-invariants to its man page

This option was added in 948994a, but the man page wasn't updated at
the time.

Author: tautschnig

doc/man/goto-instrument.1

@@ -995,6 +995,9 @@ force aggressive slicer to preserve all direct paths
 \fB\-\-apply\-loop\-contracts\fR
 check and use loop contracts when provided
 .TP
+\fB\-\-synthesize\-loop\-invariants\fR
+synthesize and apply loop invariants
+.TP
 \fB\-\-replace\-call\-with\-contract\fR \fIfun\fR
 replace calls to \fIfun\fR with \fIfun\fR's contract
 .TP