Fix python syntax

build_pop.py

@@ -21,7 +21,7 @@ parser.add_argument("--haploid", help="Make a haploid genome, instead of diploid
 parser.add_argument("-l", "--read-len", help="Generate reads having a length of LEN", type=int, default=100)
 parser.add_argument("-m", "--insert-len-mean", help="Generate inserts (fragments) having an average length of LEN",
                     type=int, default=300)
-parser.add_argument("-v", "--insert-len-sd", help="Set the standard deviation of the insert (fragment) length (%)",
+parser.add_argument("-v", "--insert-len-sd", help="Set the standard deviation of the insert (fragment) length (%%)",
                     type=int, default=30)
 
 args = parser.parse_args()