fix: typo

Author: ruifonseca

src/runtime.rs

@@ -405,7 +405,7 @@ pub fn get_method(val: Value, method_name: &str) -> Value
     static STRING_PARSE_INT: HostFn = HostFn { name: "parse_int", f: Fn2(string_parse_int) };
     static STRING_TRIM: HostFn = HostFn { name: "trim", f: Fn1(string_trim) };
     static STRING_UPPER: HostFn = HostFn { name: "upper", f: Fn1(string_upper) };
-    static STRING_LOWER: HostFn = HostFn { name: "upper", f: Fn1(string_lower) };
+    static STRING_LOWER: HostFn = HostFn { name: "lower", f: Fn1(string_lower) };
     static STRING_SPLIT: HostFn = HostFn { name: "split", f: Fn2(string_split) };
     static STRING_TO_S: HostFn = HostFn { name: "to_s", f: Fn1(identity_method) };