docs: Fix `Memory Usage` option key (#586)

docs/config/README.md

@@ -706,7 +706,7 @@ To enable it, set `disabled` to `false` in your configuration file.
 show_percentage = true
 show_swap = true
 threshold = -1
-icon = " "
+symbol = " "
 style = "bold dimmed green"
 ```
 

docs/de-DE/config/README.md

@@ -663,7 +663,7 @@ Dieses Modul ist standardmäßig deaktiviert. Setze in deiner Konfiguration `dis
 show_percentage = true
 show_swap = true
 threshold = -1
-icon = " "
+symbol = " "
 style = "bold dimmed green"
 ```
 

docs/fr-FR/config/README.md

@@ -662,7 +662,7 @@ This module is disabled by default. To enable it, set `disabled` to `false` in y
 show_percentage = true
 show_swap = true
 threshold = -1
-icon = " "
+symbol = " "
 style = "bold dimmed green"
 ```
 

docs/ja-JP/config/README.md

@@ -664,7 +664,7 @@ pure_msg = "pure shell"
 show_percentage = true
 show_swap = true
 threshold = -1
-icon = " "
+symbol = " "
 style = "bold dimmed green"
 ```
 

docs/ru-RU/config/README.md

@@ -662,7 +662,7 @@ This module is disabled by default. To enable it, set `disabled` to `false` in y
 show_percentage = true
 show_swap = true
 threshold = -1
-icon = " "
+symbol = " "
 style = "bold dimmed green"
 ```
 

docs/zh-CN/config/README.md

@@ -662,7 +662,7 @@ This module is disabled by default. To enable it, set `disabled` to `false` in y
 show_percentage = true
 show_swap = true
 threshold = -1
-icon = " "
+symbol = " "
 style = "bold dimmed green"
 ```
 

docs/zh-TW/config/README.md

@@ -661,7 +661,7 @@ pure_msg = "pure shell"
 show_percentage = true
 show_swap = true
 threshold = -1
-icon = " "
+symbol = " "
 style = "bold dimmed green"
 ```