Small doc fixes.

- mlab doesn't just do spectral computations, it also has GaussianKDE.
- The "caption" example in the plot_directive doc has a quote which gets
  incorrectly highlighted as a Python string in the rendered docs; the
  technically correct solution would be to wrap the whole thing in a `..
  code-block:: rst` but just removing the quote works just as well and
  avoids mangling the unrendered version.

Author: anntzer

lib/matplotlib/mlab.py

@@ -2,7 +2,7 @@
 Numerical Python functions written for compatibility with MATLAB
 commands with the same names. Most numerical Python functions can be found in
 the `NumPy`_ and `SciPy`_ libraries. What remains here is code for performing
-spectral computations.
+spectral computations and kernel density estimations.
 
 .. _NumPy: https://numpy.org
 .. _SciPy: https://www.scipy.org

lib/matplotlib/sphinxext/plot_directive.py

@@ -16,7 +16,7 @@
 
      .. plot:: path/to/plot.py
 
-        The plot's caption.
+        The plot caption.
 
    Additionally, one may specify the name of a function to call (with
    no arguments) immediately after importing the module::