Rename license file to satisfy go docs for nested modules (#1389)

The documentation for module `uber.go/zap/exp` is not eligible to be
rendered on pkg.go.dev as no license is detected. For example,
https://pkg.go.dev/go.uber.org/zap/exp@v0.2.0/zapslog will not render.

This is due to some possibly unfortunate behavior in the Go command that
special cases copying a `LICENSE` file from the repository for root for
any nested modules lacking a `LICENSE` file. This is problematic for Zap
as Zap's license is in a file named `LICENSE.txt`. Renaming the license
file to `LICENSE` remedies the issue.

Relevant issues:
- golang/go#43817 (comment)
- golang/go#44758 (comment)

.readme.tmpl

@@ -92,7 +92,7 @@ standard.
 
 <hr>
 
-Released under the [MIT License](LICENSE.txt).
+Released under the [MIT License](LICENSE).
 
 <sup id="footnote-versions">1</sup> In particular, keep in mind that we may be
 benchmarking against slightly older versions of other packages. Versions are

README.md

@@ -123,7 +123,7 @@ standard.
 
 <hr>
 
-Released under the [MIT License](LICENSE.txt).
+Released under the [MIT License](LICENSE).
 
 <sup id="footnote-versions">1</sup> In particular, keep in mind that we may be
 benchmarking against slightly older versions of other packages. Versions are