Add a missing verb to the description of std::process::ExitStatus::success().

"Signal termination not considered" -> "Signal termination is not considered"

The first line of the description was rewrapped so it fits into 80 characters.

Author: s3rvac

src/libstd/process.rs

@@ -799,8 +799,8 @@ impl From<fs::File> for Stdio {
 pub struct ExitStatus(imp::ExitStatus);
 
 impl ExitStatus {
-    /// Was termination successful? Signal termination not considered a success,
-    /// and success is defined as a zero exit status.
+    /// Was termination successful? Signal termination is not considered a
+    /// success, and success is defined as a zero exit status.
     ///
     /// # Examples
     ///