Fix alpha version error message (#40406)

Author: mushao999

server/src/main/java/org/elasticsearch/Version.java

@@ -257,7 +257,7 @@ public static Version fromString(String version) {
                 if (buildStr.startsWith("alpha")) {
                     assert rawMajor >= 5 : "major must be >= 5 but was " + major;
                     build = Integer.parseInt(buildStr.substring(5));
-                    assert build < 25 : "expected a beta build but " + build + " >= 25";
+                    assert build < 25 : "expected a alpha build but " + build + " >= 25";
                 } else if (buildStr.startsWith("Beta") || buildStr.startsWith("beta")) {
                     build = betaOffset + Integer.parseInt(buildStr.substring(4));
                     assert build < 50 : "expected a beta build but " + build + " >= 50";