docs: add accidentally removed markup

Signed-off-by: Philipp Burckhardt <pburckhardt@outlook.com>

Author: Planeshifter

lib/node_modules/@stdlib/stats/incr/nanvariance/README.md

@@ -30,6 +30,9 @@ The [unbiased sample variance][sample-variance] is defined as
 
 ```math
 s^2 = \frac{1}{n-1} \sum_{i=0}^{n-1} ( x_i - \bar{x} )^2
+```
+
+<!-- </equation> -->
 
 </section>