FmpDevicePkg/FmpDependencyLib: Correct ValidateDependency() documentation

FmpDevicePkg/FmpDependencyLib: Correct ValidateDependency() documentation

Modifies the return value documentation to state that the BOOLEAN
value indicates whether a given dependency expression is valid
not a capsule.

Cc: Liming Gao <liming.gao@intel.com>
Cc: Michael D Kinney <michael.d.kinney@intel.com>
Cc: Guomin Jiang <guomin.jiang@intel.com>
Cc: Wei6 Xu <wei6.xu@intel.com>
Signed-off-by: Michael Kubacki <michael.kubacki@microsoft.com>
Reviewed-by: Michael D Kinney <michael.d.kinney@intel.com>
Reviewed-by: Guomin Jiang <guomin.jiang@intel.com>
Reviewed-by: Wei6 Xu <wei6.xu@intel.com>