Update README for MFCC

Author: hqrrr

README.md

@@ -121,10 +121,10 @@ Unlike typical spectrum or spectrogram analyzers, it supports perceptual visuali
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-      <img src="data:image/svg+xml,<svg xmlns='http://www.w3.org/2000/svg' width='500' height='500'></svg>" width="100%" alt="" />
+      <img src="_pics/blank_500x500.png" width="100%" alt="blank" />
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     <td align="center" valign="top">
-      <img src="data:image/svg+xml,<svg xmlns='http://www.w3.org/2000/svg' width='500' height='500'></svg>" width="100%" alt="" />
+      <img src="_pics/blank_500x500.png" width="100%" alt="blank" />
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 </table>