[Docs][llvm-exegesis] Minor adjustments for clarity

This patch makes minor adjustments to the llvm-exegesis docs for
clarity. Particularly, an update is made to the list of snippet
annotations to list the correct number of annotations that was not
updated when the docs were originally updated for the snippet address
annotation. In addition, this patch changes a decimal value for the
snippet memory annotation example for an explicit hex value to emphasize
that the LLVM-EXEGESIS-MEM-DEF annotation takes a hex value for the
memory value.