Fix example in doc for fetch_by_gene_stable_id().

modules/Bio/EnsEMBL/DBSQL/SliceAdaptor.pm

@@ -993,7 +993,7 @@ sub fetch_by_transcript_id {
   Arg [2]    : (optional) int $size
                The length of the flanking regions the slice should encompass
                on either side of the gene (0 by default)
-  Example    : $slc = $sa->fetch_by_transcript_stable_id('ENSG00000012123',10);
+  Example    : $slc = $sa->fetch_by_gene_stable_id('ENSG00000012123',10);
   Description: Creates a slice around the region of the specified gene.
                If a context size is given, the slice is extended by that
                number of basepairs on either side of the gene.