Use "number" instead of "digit" when explaining Cargo's semver

Digit is technically incorrect here, as it would imply that Cargo treats 0.10.0 and 0.11.0 as semver compatible (because they have the same leftmost non-zero *digit*, even though they do not have the same leftmost non-zero *number*).

Author: nicoburns

src/doc/src/reference/specifying-dependencies.md

@@ -22,7 +22,7 @@ time = "0.1.12"
 The string `"0.1.12"` is a version requirement. Although it looks like a
 specific *version* of the `time` crate, it actually specifies a *range* of
 versions and allows [SemVer] compatible updates. An update is allowed if the new
-version number does not modify the left-most non-zero digit in the major, minor,
+version number does not modify the left-most non-zero number in the major, minor,
 patch grouping. In this case, if we ran `cargo update -p time`, cargo should
 update us to version `0.1.13` if it is the latest `0.1.z` release, but would not
 update us to `0.2.0`. If instead we had specified the version string as `1.0`,