typo fixed in code snippet for undeclared variable

variable `axs` was not defined in the code snippet of `Sharing x per column, y per row` figure, rather it was defined in some earlier snippets. The intended use here is applying `label_outer()` on the axes of the figure in the current snippet.

Author: rifatrakib

examples/subplots_axes_and_figures/subplots_demo.py

@@ -176,7 +176,7 @@
 ax3.plot(x + 1, -y, 'tab:green')
 ax4.plot(x + 2, -y**2, 'tab:red')
 
-for ax in axs.flat:
+for ax in fig.get_axes():
     ax.label_outer()
 
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